1. Field of the Invention
This invention relates to a circuit for stabilizing an RF (radio-frequency) amplifier such as a microwave amplifier.
2. Description of the Prior Art
A typical microwave amplifier includes an FET, and input and output matching circuits connected to the FET. The drain of the FET is connected to a bias power supply so that the FET can be fed with a bias current from the bias power supply. An inductor is connected between the drain of the FET and the bias power supply to prevent a microwave signal from leaking toward the bias power supply.
Generally, in such a microwave amplifier, the source of the FET is grounded via an inductor which enables a reduced noise figure at an operating frequency, that is, a frequency of a signal handled by the amplifier. As the inductance of the noise-reducing inductor increases, the noise figure of the amplifier decreases. However, the amplifier tends to oscillate at a high inductance of the noise-reducing inductor.
To prevent such oscillation corresponding to the amplifier and thereby increase the stability thereof, it is known to connect a damping resistor to the FET. A book entitled "MICROWAVE TRANSISTOR AMPLIFIERS", written by Guillermo Gonzales and published by PRENTICE-HALL, INC., teaches that the resistance of the damping resistor should be within a given stability circle (a given constant-conductance circle) in a Smith chart.
There are three examples of such a damping resistor. The first example is a series resistor connected to the gate of the FET. The second example is a series resistor connected to the drain of the FET. The third example is a shunt resistor connected between the drain and the source of the FET.
The admittance of the damping resistor is fixed independent of frequency. Therefore, in some cases, the damping resistor tends to have a excessively large conductance with respect to a signal in a frequency band predetermined for the amplifier. The excessively large conductance results in a reduction in the gain of the amplifier and an increase in the noise figure of the amplifier.